Compared to some forms of racing, caster and camber settings are evaluated a little differently for circle track racing. Because we usually only turn left (as opposed to street driving or road racing), our front-end design is much different and asymmetrical (differing from side to side). The knowledge of what constitutes the optimum design and what the car needs has increased over the past few years.
It is important to know how to properly measure the amount of caster and camber in your race car. From my experience, most teams know little about the proper procedure for determining the amount of each that exists in the front-end geometry of their race cars.
Caster Defined Caster is a design condition that, in addition to the spindle kingpin angle, serves to cause a wheel to want to track straight ahead as a product of weight being applied to the wheel structure. A common example is a bicycle front wheel and fork assembly. The tube on which the handlebars are mounted is placed in a set of bearings above the fork. From a side view, this tube is angled so the bottom bearing is ahead of the top bearing. If we turn the front wheel to 90 degrees from the direction of travel, it will want to return to straight ahead because of the effect of caster. The same effect is present in the front wheel assemblies of our race car.
What Caster Does To ease the amount of effort it takes to turn the wheel in our race cars, caster split was introduced into the design. Split means that we set different caster amounts into each wheel assembly so the car will want to turn to the left and thereby reduce the amount of effort it takes for the driver to hold the steering wheel when negotiating the turns. Proper split for circle track racing means the left-front (LF) wheel will have less positive caster than the right-front (RF) wheel. In some cases, teams have been known to set negative caster in the LF wheel and positive caster in the RF wheel.
To measure caster in each wheel, we use a caster/camber gauge. This tool attaches to the wheel hub. To check the amount of caster, we need to follow these instructions:
1. Attach the caster/camber gauge to the RF wheel hub first.
2. Turn the steering wheel to the right so the RF wheel has turned exactly 20 degrees.
3. Level the gauge and set the adjustable caster bubble vial so the bubble is at the zero mark on the caster side of the tool.
4. Turn the steering wheel to the left so the RF wheel is turned past straight ahead and ending up at left from straight ahead by 20 degrees.
5. Again, level the gauge, note the location of the bubble on the scale, and record the amount of caster in the RF wheel.
6. While the wheels are still turned left 20 degrees, remove the caster/camber gauge and place it onto the LF wheel hub.
7. Level the gauge and set the bubble on the caster gauge to zero.
8. Turn the steering wheel to the right past straight ahead until the LF wheel is turned 20 degrees to the right of straight ahead.
9. Level the gauge and read the caster gauge to see how much caster is in the LF wheel.
Adjusting Caster To adjust the amount of caster in each wheel, you will need to move the upper ball joints fore or aft. To increase the amount of positive caster, move the top ball joint toward the rear of the car. Some cars have slots cut into the upper chassis mounts for this purpose. If you have permanently attached vertical mounting plates for attaching the upper control arms, you can vary the amount of shim spacing for each of the bolts that attach the control arm to the chassis. Wider spacing at the front bolt (control arm shaft inside of the mounting plate) will move the upper ball joint to the front, creating less caster at that wheel and so on. This, however, is not the preferred method. Once you have established the exact caster amounts for each wheel using the above method (if not using slotted control arm shafts), you should order an upper control arm that has the ball joint offset to give the correct amount of caster at each wheel. That way, you can use the same shim spacing for each mounting bolt to connect the upper control-arm shaft to the chassis.
In today's complex world of...
In today's complex world of chassis setup, we cannot overlook theimportance of properly adjusting our car's front geometry settings.Caster and camber are a major part of that equation.
Positive caster in the front...
Positive caster in the front wheel assembly is created when the balljoints are offset (from a side view) so the upper ball joint is fartherto the rear of the car than the bottom ball joint. The degree of casteris related to the angle in degrees that a line through the ball jointsforms from a vertical line.
Set the bubble at zero on...
Set the bubble at zero on the caster side of the gauge. Some teams useone edge of the bubble so reading the angle is more accurate than tryingto estimate the center of the bubble.
To measure on the floor to...
To measure on the floor to get exactly 20 degrees of steering, cut a30-inch piece of 1-by-6-inch wood or similar straight, flat piece andlay it against the tire. Mark midway on the wood (15 inches) and linethat up with the hub. Mark the outside corner of each end of the wood.Turn the steering wheel until the ends have moved 53/16 inches and youwill have turned the wheel 20 degrees.
Some cars have the upper control-arm...
Some cars have the upper control-arm mounts cut with a slot that makesadjusting caster easy. Some upper control arms are also slotted for thesame reason. If your mounting plates or control arms do not offer thismethod of adjusting caster, you can angle (from a top view) the controlarm shaft by moving shims front to rear. This is not the ideal method,but will work until we can find the right caster split that suits thedriver. We can then order offset control arms that will locate the upperball joint for proper caster without needing to angle the shaft.
Move camber adjustment shims...
Move camber adjustment shims back and forth from each mounting bolt, orslide the control arm shaft if it is slotted, to adjust the amount ofcaster in each wheel. This will move the upper ball joint forward (toreduce caster) or to the rear (to add caster).
After you have leveled the...
After you have leveled the gauge, read the center of the bubble on thecamber side of the gauge to get your wheel camber setting. Make sure thecar is at ride height with all of the weight in the car including thedriver. Air the tires to operating pressures. These settings may changeonce the car is raced and the tire temperatures have been evaluated.
How Much Caster Split Normal caster splits for most short track asphalt applications are around 2 to 4 degrees of difference. The LF caster might be 1-2 degrees and the RF caster might be 3-5 degrees. Less steering effort is needed on higher banked tracks, so less caster is needed. Also, the tighter the turn radius, the more caster split is needed. Driver preference plays a big role in getting the caster split right for your application.
Camber Defined Camber is when a wheel is tilted, from a front view, so that the top of the tire is either closer to the centerline of the car or farther from it. Negative camber is when the top of the tire is closer to the center of the car than the bottom of the tire. Positive camber is when the opposite is true, i.e., the top of the tire is farther away from the center of the car than the bottom of the tire.
Circle Track Camber In circle track racing, we use positive camber on the LF wheel of the car and negative camber on the RF wheel. We can easily check the amount of camber by using a caster/camber gauge and reading the amount directly on the camber bubble vial.
We have learned some interesting and important aspects of tire camber for short track racing. We have always known a racing tire will flex under the stress of cornering and the tread will move and roll under the wheel when the extreme forces associated with cornering are present as we turn left. Different brands of tires have different stiffness of sidewall construction. Tire temperatures tell us more about how much static camber we need than anything else. The overall goal is that we need the tire contact patch to be relatively flat on the racing surface at mid-turn in order for the tire to be able to provide the maximum amount of traction it is capable of giving. This is often referred to as the maximum "footprint."
Tire temperatures can alert us to improperly set static cambers. A front tire that is hotter on the inside edge (side toward the inside of the racetrack) usually has too much positive camber (in the case of a LF wheel) or too much negative camber if it is the RF wheel.
Camber Change The cambers will change as the car dives and rolls as it enters and negotiates a turn. True camber change is a product of both chassis dive and chassis roll. Gone are the days when we would jack up the wheel and measure how many degrees the camber changed in each inch of bump. Those numbers are only part of the front end dynamics answer and don't really tell us enough. Chassis roll has an effect that adds or subtracts from what dive does. We really need to know where the dynamic camber ends up after the car dives and rolls, just like it does in the turns.
The LF always loses a lot of its static camber, so we need to allow for that in setting the amount of static camber. Generally, if we end up with between 1/2 to 1 degree of positive camber at the LF wheel after the car dives and rolls, then that tire will have the dynamic (after the forces are applied) camber that it needs for the weight that remains after the weight has transferred in the turns.
Right-Front Camber Change The RF camber change is a little different. We can design our car so the RF camber does not change after dive and roll. This is exactly what that tire wants for most short track applications. As we enter the turn, the RF tire takes a set fairly quickly. If the camber continues to change after that initial set, the tire will give up traction and the car will usually push. The right upper control-arm angle mostly controls the RF camber change, so we try to work with that control arm angle. Once we have the proper camber change (zero), we leave that angle alone as we further design our front end for roll center location.
Spindle Height Spindle height affects the amount of camber change at each wheel. The taller the spindle, the less camber change that will occur. Trends that have taken place in the past 10 years or so have resulted in excess camber change due to the use of shorter spindles. That trend seems to be reversing as car builders move toward using taller spindles.
Measuring Camber Change We can measure camber change by several different methods. In the shop, we can set the chassis ride heights just as they would be at mid-turn on the racetrack and then directly measure the camber at each wheel. To do this, we will need to know the shock travel at mid-turn, which is very hard to estimate. If we look at the shock travel indicators on the shaft of the shock, it always tells us total shock travel, which includes braking, going over bumps, banking changes such as exiting the race track and driving down onto the apron (this could be quite a lot of LF shock travel at some high-banked racetracks), or something as simple as steering the car back and forth to warm the tires before running hot laps.
The easiest and most accurate way to look at true camber change is to use a computer geometry software program. There are several good ones on the market today. You enter the height and width measurements (and fore and aft when using a three-dimensional geometry program) for each ball joint and the chassis pickup points as well as the static camber amounts. These programs let you enter estimated dive and roll numbers. When you calculate these numbers, the dynamic camber at each wheel is shown. That way, you will know exactly what is happening with your wheel cambers.
Remember that caster settings are mostly adjusted for driver preference and comfort, and camber settings are important so the front end will have the maximum amount of tire footprint and traction to use to turn the car at mid-turn. Many driver-related problems come from camber change problems. Often, a car with a serious push can be helped by analyzing and adjusting the static camber, as well as knowing the camber change. Overlook these two important aspects of front-end geometry and your performance may suffer.
From a driver's view, the...
From a driver's view, the positive LF camber causes the top of that tireto lean out away from the centerline of the car. The opposite is true ofthe RF tire that has negative camber. On that wheel, the top of the tireleans in toward the centerline.
As the chassis dives, the...
As the chassis dives, the upper ball joints are pulled in toward thecenterline of the car. This causes a movement toward negative camber inboth front wheels. The greater the angle of the upper control arms, themore change that occurs with the camber from chassis dive. But, this isnot all that is happening.
Besides chassis dive, when...
Besides chassis dive, when we turn the car left, the chassis also rolls.As the chassis pick-up points move during roll, the upper ball jointsmove to the right. This causes the LF wheel to further lose cambertoward the negative and the RF wheel to move toward positive camber. Ifwe combine the effects of dive and roll, we see where the LF wheel willlose in both dive and roll where the RF dynamic camber will be a productof combining two movements that are in opposite directions. That is whywe can achieve a net zero change in camber at the RF wheel.
The height of the spindle...
The height of the spindle affects the amount of camber change at eachfront wheel. On the left is a short spindle, and we can see that for thesame 1-inch lateral movement of the upper ball joint, the short spindlechanges camber angle by roughly one more degree than the taller spindleshown on the right.